Pilot-wire protection for transmission lines



7, 1936- L'. B. LE VESCONTE 2,027,237

PILOT WIRE PROTECTION FOR TRANSMISSION LINES Filed Oct. 11, 1934 INVENTOR (24% 4 lesrerBlelfesconfie.

ATTORNEY Patented Jan. 7, 1936 UNITED STATES PATENT OFFICE PILOT-WIRE PROTECTION FOR TRANSMISSION LINES Pennsylvania Application October 11, 1934, Serial No. 747,914

14 Claims.

This invention relates to a certain system of relay-protection for alternating-current transmission lines, known as pilot-wire protection, and it particularly relates to such a system in which it is possible to use, at each end of the linesection being protected, current-diiferential relays which will operate only when the currentunbalance at the relay becomes more than a predetermined percentage of the total current in the transmission line.

Heretofore, in one pilot-wire system using ourrent-ba1ance relays, it has been necessary to utihas a relatively large artificial impedance load on the line-current transformers at each end of the protected line-section, said load being considerably larger than the load imposed by the pilot-wire and the coils of the relays, so as to convert the current-transformer circuit into a voltage circuit, with a variable voltage impressed over the pilot-wires, rather than a variable current. This imposes a very considerable extra load on the current-transformers, and is objectionable from other standpoints, as well. In another pilot-wire system heretofore proposed, it has been necessary to utilize the differential current-relay at only one end of the pilot wire, thus requiring the trip-circuit to be carried over additional pilot wires to the other end.

An essential feature of my invention is the provision of means for circulating over the pilot wire, or for transmitting through some other communicating channel, a current which is a predetermined composite function of the linecurrents at both ends of the protected linesection.

An important feature of my invention is that it permits the utilization of a percentage-unbalance relay at each end of the protected linesection, both of these relays responding to the percentage or ratio represented by the difference between the line-currents at the two ends of the protected line-section divided by the line current. The denominator of this ratio may be represented either by the value of the line-current at only one end, or by the sum of thelinecurrents at the two ends of the protected linesection.

This type of relay is much more desirable than relay which operates upon a predetermined amount (rather than a predetermined percentage) of unbalance between the line-currents at the two ends. The reason for this is that it is necessary that the relay should not operate upon the occurrence of extremely heavy fault-currents which flow all the way through the line, entering at one end and leaving at the other, so that the two line-currents at the opposite ends of the protected line-section are equal, and also in phase. Theoretically, the current-unbalance at the relay should be equivalent to the difference 5 of the currents in the two ends of the protected line-section, which would be zero in such a case, or in any case unless there should be a shortcircuit at some intermediate point between the ends of the protected line-section. Actually, such perfect balance is not possible, particularly under high-current conditions, which may cause the. current transformers to approach saturation in different degrees, resulting in discrepancies in the current-transformer ratios at the two ends 15 of the protected line-section. The significance of this is that a current-unbalance relay which operates in response to a predetermined amount of unbalance may be operated when an extremely large current flows through the protected linesection, whereas a current-differential relay which operates upon a predetermined percentage of unbalance will not be affected by the difference in the characteristics of the currenttransforiners or by any other causes resulting in 5 a comparatively very small current-unbalance as compared to the actual line-currents flowing at the moment. While my invention is not limited to the use of percentage-unbalance relays, it is, nevertheless, an important advantage of my in- 0 vention that it permits the use of such relays at both ends of the protected line-section.

With the foregoing and other objects in view, my invention consists in the circuits, systems, methods and apparatus hereinafter described and claimed and illustrated in the accompanying drawing, wherein:

Figure 1 is an explanatory schematic View illustrating the general principles and operation of my invention; and

Figs. 2, 3 and 4 are similar views indicating different forms of embodiment thereof.

Fig. 1 illustrates my invention as applied to a single-phase transmission line' 5 having circuit breakers 6 and l at the respective ends thereof. At each end there is provided a relay station at which is located a differential-current relay 8 and 9, respectively. These relays are shown as being of the current-differential type, each comprising an operating coil 0 and a restraining coil R, said coils exerting torques in opposite directions on a movable element, in this case illustrated as a balanced beam 19, carrying a movable contact member ll. When the magnitude of the current in the operating coil exceeds a predetermined precentage of the magnitude of the current in the restraining coil, the contact element H is closed and serves to energize the tripping circuit of the circuit breaker 6 or i, as the case may be.

In the embodiment of my invention illustrated in Fig. 1, I provide a current-transformer l3 and it, respectively, at each end of the protected line 5. The line-current is indicated at I and the secondary currents in the current-transformers i3 and M are indicated by I1 and I2, respectively. The current I1 of the currenttransformer i3 is passed first through the restraining coil R, and thence to an equalizer means in th shape of the mid-tap of a current transformer 55, wherein two branch circuits !5 and ill are provided, each traversed by I1, as indicated by the arrows.

The end of the branch-circuit I6 is connected to two circuits, to wit, an operating-coil circuit 5 8, which contains the operating coil 0, and a pilot-wire or communicating-channel circuit 99 which, in the form of embodiment shown in Fig. l, is the actual pilot wire itself, said pilot wire extending between the two relaying stations at the opposite ends of the protected line 5. The impedance of the pilot wire is represented diagrammatically as being lumped at a single point and designated by the lumped impedance Z.

The other branch-circuit I? is also connected to two circuits, to wit, the other end of the aforementioned operating-coil circuit I8, and a dummy-impedance circuit which contains a dummy impedance indicated by means of a current transformer and a lumped impedance 22, the resultant eifect of which is equivalent to one-half of the impedance Z of the pilot-wire channel. The other ends of the dummy-impedance circuit 2!] and of the pilot-wire circuit l9 are connected to the return-circuit 23 of the current-transformer i3.

Similar equipment is provided at the opposite end of the protected line 5 in Fig. 1, the connections being such that the currents circulated over the pilot-wire E9, or transmitted from one end of the line-section to the other by means of any equivalent communicating-channel, shall be equal to substantially (I1+I2).

It will be noted that the return-current I1, flowing in the conductor 23 back to the currenttransformer i3, is divided between the dummyilnpedance circuit 26 and the pilot-Wire circuit in, the latter comprising the first pilot-wire 24, the dummy-impedance circuit til at the opposite end of the line-section, the operating coil 0 of the difierential current relay 9 at said opposite end, the second pilot-wire conductor 25 which is represented as carrying the lumped pilot-wire impedance Z, and the operating coil 0 of the differential current-relay 3 at the station con taining the current-transformer E3. The pilotwire current does not pass through the equalizer means 55 because said equalizer means has a very much higher impedance to through-currents than the impedance of the operating-coil circuit 58 which is shunted around it. This equalizer means permits the ready flow of current, only when equal and opposite currents are flowing in the two halves l6 and I1 thereof.

As compared to the impedance Z of the pilotline, the impedances of the operating coils 0 may be regarded as negligible, or if not negligible, they may be considered as being lumped in the impedance represented at Z, and hence the impedance of the pilot-wire circuit, including the dummy-impedance circuit 2E3 at the opposite end of the line-section, is 3/25. It is, of course, apparent, therefore, that three-fourths of the relay current I1 flows through the dummy impedance circuit 20 and one-fourth of it flows through the pilot-channel.

The relay current I2 originating at the other end of the protected line-section 5 is divided in a. similar manner, as indicated by the arrows in Fig. l, with the result that the pilot wire carries an operating current equal approximately to ,(I1+I2), whereas the two operating coils O of the two relays 8 and 9 at the opposite ends of the protected line-section are each traversed by currents ,(I1.Iz).

Fig. 2 illustrates the connections to the operating and restraining coils O and R of the relays for protecting a three-phase line-section 38 by means of a bank of star-connected current-transformers 3| and 32, respectively, at the opposite ends of the protected line-section. In this case, the pilot wires 34 are separate from the pilotwire circuits 35, 35'; 3t, 38'; 37, 37; being connected by means of step-down current transformers 33 which make it possible to cause the currents circulated in the pilot wires 34 to be much smaller than the corresponding currents in the pilot wire-circuits, such as 35. In other respects, the operation of the three-phase system shown in Fig. 2 is as indicated in connection with Fig. 1, it being understod that three separate difierential-current relays are utilized at each end of the protected line-section, so that if a predetermined differential current-unbalance is obtained in any one of the three phases, suitable protective measures, such as the tripping of a circuit breaker (not shown in Fig. 2), or

other protective means, may be set in operation.

Fig. 3 shows a modification of my three-phase protective scheme in which the current-transformers 3i are connected in delta, rather than being connected in star or Y formation. With this connection, the current-difierential relays are designed to respond to phase-to-phase currents rather than phase-to-neutral currents. vantage of this arrangement, as set forth in an application of W. A. Lewis, Serial No. 650,518, filed January 6, 1933, for Pilot-wire protection, is that the zero-sequence component of the line-currents in the protected line-section 30 are caused to circulate harmlessly in the delta-circuit of the current-transformers 3i, and are kept out of the pilot wires 3 thereby avoiding certain trouble from inductive interference in the pilot wires 34 in the event of single-phase ground-faults.

The modification shown in Fig. 3 also includes a different type of equalizer means, consisting, in this case, of two small impedance devices 4i and 32, taking the place of the two branch-circuits i6 and il of the balance-coil current-transformer B5 in Fig. 1. In Fig. 3, by way of illustration, I have indicated the possibility of separating the restraining coil of each relay into two parts marked Bi and R2, respectively, and placing those two parts in the two branch-circuits d! and 32, respectively. It will be noted that the two branchcircuits 4! and .2 are connected together at one end, at what constitutes the middle point 33 of the equalizer device, and are connected at the other end by the relatively impedance of the operating coil 0. The impedances 4i and Q2 of the equalizer device are made sufiiciently large, relative to the impedance of the operating coil 0, so as to exclude practically all of the pilotwire current from the impedances 6i and 42,

One adand these impedances 4| and 42 are made substantially equal to each other so that the currents from the current-transformer 3| at the station in question are substantially equally divided.

In the embodiments of my invention shown in Figs. 1 to 3, it will be noted that the restraining coils R, or RI and R2, as the case may be, are energized in accordance with the line-current at the station where the relay is located. This may be taken as being sufiiciently accurately responsive to the line-current. However, if it is desired to totalize the line-currents flowing at the opposite ends of the protected line-section, this may readily be done by placing the restraining coil R in the pilot-wire circuit such as 35", as indicated in Fig. 4, thus causing the restraining coil to be energized with a current which is proportional to the sum of the line-currents at the two ends of the protected line-section.

In the specification and claims, in referring to the sum or the difference of the line-currents at the two ends of the protected line-section, I refer to the normal current-flow, with current being considered positive at both ends of the linesection, notwithstanding the fact that it flows into the line-section at one end and out of it at the second end. If the current reverses at the second end, as a result of an internal fault within the protected zone, the in-fiowing current at that end is then regarded as negative in value, and if it is equal to the iii-flowing current at the firstmentioned end of the protected line-section, the resultant current flowing in the pilot wire, or otherwise transmitted by any suitable communicating channel, is equal to zero.

By my protective pilot-wire system, I impose only a small additional burden on each linecurrent transformer, as represented by the impedance Z, which is small as compared to the burden necessarily imposed by the pilot-wire and the operating and restraining coils of the relay. Moreover, by reason of the fact that I utilize percentage-dilferential relays, I provide a system which is not sensitive at all to slight maladjustments of the current-transformer ratios, or slight maladjustments in the values of the dummy-circuit impedances or in the exactly equal division of the relay-currents in the two branch-circuits i6 and II or 4| and 42. At the same time, my relay system is very sensitive to current-unbalance caused by the simultaneous in-flow of currents at the two ends of the protected line-section, so that positive tripping, or other faul -indication, may be obtained at both ends of the protected line-section, no matter where the fault is, and no matter which end has the larger fault-current.

While I have illustrated my invention in several different forms of embodiment thereof, it will be understood that such illustration is not exhaustive of the variations to which my invention is susceptible, without departing from the essential principles thereof. I desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language and the prior art.

I claim as my invention:

1. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, means for providing a communicating channel between said stations, line-current-responsive relay-current means at one relaying station for deriving a relaying current I1 from said line, line-current-responsive relay-current means at the other relaying station for deriving a relaying current I2 from said line, said relaying currents I1 and I2 being substantially equal in phase and magnitude when no current is withdrawn from or supplied to the protected line-section at any point intermediate of its ends, equalizer means at the first relaying station for providing two branch-circuits each traversed by substantially I1, equalizer means at the second relaying station for providing two branchcircuits each traversed by substantially M212, an operating-coil circuit and a communicating-channel circuit at each station, connected to the end of one of said branch-circuits, the other end of the operating-coil circuit at each station being connected to the end of the other branch circuit, a dummy-impedance means at each station, also connected to the end of the last-mentioned branch-circuit, means including said communicating channel for linking together the communicating-channel circuits at the two ends of the protected line-section, the equalizer means at each station being of such nature as to substantially exclude the flow therein of currents from the other end of the line-section, the dummy-impedance circuit at the first-mentioned station including such impedance as to cause substantially of the relay current I1 to flow therein and to cause substantially A,, of the relaying current I1 to flow in the communicating-channel circuit at said station, the dummy-impedance circuit at the second-mentioned station including such impedance as to cause substantially of the relaying current I2 to flow therein and to cause substantially of the relaying current I2 to flow in the communicating-channel circuit at said station, whereby each of the communicating-channel circuits is traversed by a current of substantially (11-1-12), and each of the operating-coil circuits is traversed by a current of substantially 1 A (I1-I2), and a differentialcurrent. relay at each station having an operating coil energized in response to the operating-coll circuit at its station.

2. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, means for providing a communicating channel between said stations, line-current-responsive relay-current means at each station for deriving relaying currents from said line, means for deriving, from said relaying currents, and transmitting through said communicating channel, a current which is a substantially equal function of the relaying cu;- rents of both ends of the protected line-section, and a differential-current relay-means at each end of the protected line-section, associated with the foregoing apparatus, each of said differential-current relay-means having an operating coil connected to respond differentially to the linecurrents at both ends of the protected line-section, and having a restraining coil connected to be responsive to the magnitude of the line-current prevailing at the moment.

3. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, means for providing a communicating channel between said stations, line-current-responsive relay-current means at each station for deriving relaying currents from said line, means for deriving, from said relaying currents, and transmitting through said communicating channel, a current which is a substantially equal function of the relaying currents of both ends of the protected line-section. and a differential-current relay-means at each end of the protected line-section, associated with the foregoing apparatus, each of said difierentialcurrent relay-means having an operating coil connected to respond differentially to the linecurrents at both ends of the protected line-sec tion, and having a restraining coil connected to be responsive to the magnitude of the line-current at its own station.

4. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, means for providing a communicating channel between said stations, line-current-responsive relay-current means at each station for deriving relaying currents from said line, means for deriving, from said relaying currents, and transmitting through said communicating channel, a current which is a substantially equal function of the relaying currents of both ends of the protected line-section, and a differential-current relay-means at each end, of the protected linesection, associated with the foregoing apparatus, each of said differentialcurrent relay-means having an operating coil connected to respond differentially to the line-currents at both ends of the protected line-section, and having a restraining coil connected to respond cumulatively to the line-currents at both ends of the protected line-section.

5. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section there-oi, means for providing a communicating channel between said stations, line-currentresponsive relay-current means at each station for deriving relaying currents I1 and I2, respectively, from opposite ends of said protected line-section, said relaying currents I1 and I2 being substantially equal in phase and magnitude when no current is withdrawn from or supplied to the protected line-section at any point intermediate of its ends, means for deriving, from said relaying currents, and transmitting through said communicating channel, a current proportional substantially to (I1+I2), means at each end of said protected line-section, for responding to said transmitted (I1+I2) current and for responding substantially twice as strongly to the local relaying current (I1) or (I2), as the case may be, so as to obtain a resultant response which is substantially proportional to :(Iz-Ir), and a diiierential-current relay, at each station, provided with operating coil means having said i-(Iz-h) response, and provided with restraining-coil means connected to be responsive to the magnitude of the linecurrent prevailing at the moment.

6. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, means for providing a communicating channel between said stations, line-current-respcnsive relay-current means at each station for deriving relaying cur-- rents I1 and I2, respectively, from opposite ends of said protected line-section, said relaying currents I1 and I2 being substantially equal in phase and magnitude when no current is withdrawn from or supplied to the protected line-secticn at any point intermediate of its ends, for deriving, from said relaying currents, and transmitting through said communicating channel, a current proportional substantially to (Ii-H2), means at each end of said protected line-section, for responding to said transmitted (Ir-i-Iz) cur rent and for responding substantially twice as strongly to the local relaying current (-11) or (-12), as the case may be, so as to obtain a resultant response which is substantiallyproportional to :(IzI1), and a differential-current relay, at each station, provided with operatingcoil means having said :(Iz-Ir) response, and provided with restraining-coil means connected to be responsive to the magnitude of the linecurrent at its own station.

7. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, means for providing a communicating channel between said stations, line-current-responsive relay-current means at each station for deriving relaying currents I1 and 12, respectively, from opposite ends of said protected line-section, said relaying currents I1 and 12 being substantially equal in phase and magnitude when no current is withdrawn from or supplied to the protected line-section at any point intermediate of its ends, means for deriving, from said relaying currents, and transmitting through said communicating channel, a current proportional substantially to (Ir-i-Iz), means at each end of said protected line-section, for responding to said transmitted (I1+I2) current and for responding substantially twice as strongly to the local relaying current (I1) or (-12), as the case may be, so as to obtain a resultant response which is substantially proportional to :(IzI1), and a differential-current relay, at each station, provided with operatingcoil means having said :(I2I1) respon e, and provided with restraining-coil means responsive to the (Ii-I-I2) current.

8, Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, pilot-wire means between said stations, 1ine-currentresponsive relay-current means at one relaying station for deriving a relaying current I1 from said line, line-current-responsive relay-current means at the other relaying station for deriving a relaying current I2 from said line, said relaying currents I1 and I2 being substantially equal in phase and magnitude when no current is withdrawn from or supplied to the protected linesection at any point intermediate of its ends, equalizer means at the first relaying station for providing two branch-circuits each traversed by substantially 11, equalizer means at the second relaying station for providing two branchcircuits each traversed by substantially 12, an operating-coil circuit and a pilot-wire circuit at each station, connected to the end of one of said branch-circuits, the other end of the operatingcoil circuit at each station being connected to the end of the other branch-circuit, a dummyimpedance means at each station, also connected to the end of the last-n1entioned branch-circuit, means including said pilct-wire-means for linking together the pilot-wire circuits at the two ends of the protected line-section, the equalizer means at each station being of such nature as to substantially exclude the flow therein of currents from the other end of the line-section, the dummy-impedance circuit at the first-mentioned station including such impedance as to cause substantially of the relaying current I1 to flow therein and to cause substantially A of the relaying current I1 to rlow in, the pilot wire-circuit at said station, the dummy-impedance circuit at the second-mentioned station including such impedance as to cause substantially of the relaying current I2 to flow therein and to cause substantially A of the relaying current I2 to fiow in the pilot-wire circuit at said station, whereby each of the pilot-wirecircuits is traversed by a current of substantially A(I1+Iz), and each of the operating-coil circuits is traversed by a current of substantially i (I1I2), and a differential-current relay at each station having an operating coil energized in response to the operating-coil circuit at its station.

9. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, pilot-wire means between said stations, line-current-responsive relay-current means at each station for deriving relayingcurrents from said line, means for deriving, from said relaying currents, and transmitting through said pilot-wire means, a current which is a substantially equal function of the relaying currents of both ends of the protected line-section, and a difierential-current relay-means at each end of the protected line-section, associated with the foregoing apparatus, each of said difierential-current relay-means having an operating coil connected to respond differentially to the line-currents at both ends of the protected line-section, and having a restraining coil connected to be responsive to the magnitude of the line-current prevailing at the moment.

10. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, pilot-wire means between said stations, line-current-"esponsive relay-current means at each station for deriving relaying currents from said line, means for deriving, from said relaying currents, and transmitting through said pilot-wire means, a current which is a substantially equal function of the relaying currents of both ends of the protected line-section, and a differential-current relay-means at each end of the protected line-section, associated with the foregoing apparatus, each of said differential-current relay-means having an operating coil connected to respond differentially to the line-currents at both ends of the protected line-section, and having a restraining coil connected to be responsive to the magnitude of the line-current at its own station.

11. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, pilot-wire means between said stations, line-current-responsive relay-current means at each station for deriving relaying currents from said line, means for deriving, from said relaying currents, and transmitting through said pilot-wire means, a current which is a substantially equal function of the relaying currents of both ends or the protected line-section, and a difierential-current relay-means at each end of the protected line-section, associated with the foregoing apparatus,

each. of said differential-current relay-means having an operating coil connected to respond differentially to the line-currents at both ends of the protected line-section, and having a restraining coil connected to respond cumulatively to the line-currents at both ends of the protected line-section.

12. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, pilotwire means between said stations, line-currentresponsive relay current means at each station for deriving relaying currents I1 and I2, respectively, from opposite ends of said protected linesection, said relaying currents I1 and I2 being substantially equal in phase and magnitude when no current is withdrawn from or supplied to the protected line-section at any point intermediate of its ends, means for deriving, from said relaying currents, and transmitting through said pilotwire means, a current substantially proportional to (11-1-12), means at each end of said protected line-section, for responding to said transmitted (11+I2) current and for responding substantially twice as strongly to the local relaying current (-11) 01' (I2), as the case may be, so as to obtain a resultant response which is substantially proportional to i(l'2I1), and a differential-current relay, at each station, provided with operating-coil means having said ;t(I2I1) response, and provided with restraining-coil means connected to be responsive to the magnitude of the line-current prevailing at the moment.

13. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, pilotwire means between said stations, line-currentresponsive relay-current means at each station for deriving relaying currents I1 and I2, respectively, from opposite ends of said protected linesection, said relaying currents I1 and 12 being substantially equal in phase and magnitude when no current is withdrawn from or supplied to the protected line-section at any point intermediate of its ends, means for deriving, from said relaying currents, and transmitting through said pilot-wire means, a current substantially proportional to (I1+I2), means at each end of said protected line-section, for responding to said transmitted (I1+I2) current and for responding substantially twice as strongly to the local relaying current (-11) or (-42), as the case may be, so as to obtain a resultant response which is substantially proportional to (I2I1), and a ditferential-current relay, at each station, provided with operating-coil means having said (I2I1) response, and provided with restraining-coil means connected to be responsive to the magnitude of the line-current at itsown station.

14. Protective means for an alternating-current line having two distant relaying stations at opposite ends of a line-section thereof, pilotwire means between said stations, line-currentresponsive relay-current means at each station for deriving relaying currents I1 and I2, respectively, from opposite ends of said protected lineseetion, said relaying currents I1 and I2 being substantially equal in phase and magnitude when no current is withdrawn from or supplied to the protected line-section at any point intermediate of its ends, means for deriving, from said relaying currents, and transmiting through said pilotwire means, a current substantially proportional to (11+I2), means at each end of said protected line-section, for responding to said transmitted (I1+I2) current and for responding substantially twice as strongly to the local relaying current (-11) or (-12), as the case may be, so as to obtain a resultant response which is substantially proportional to 1*:(12-11), and a differential-current relay, at each station, provided with operating-coil means having said i(I2-I1), response, and provided with restraining-coil means responsive to the (11+I2) current.

LESTER B. LE VESCON'I'E. 

